Synthetic soils for horticulture (i. e., solid substrates for plant support) include two general categories--inert and active. Inert substrates are commonly used in nutriculture (e. g., hydroponics) and are designed to provide mechanical support, proper root aeration and drainage. Quartz sand is a good example of an inert soil. Plant nutrients are added separately as, for example, liquid fertilizers such as Hoagland's solution. Soils which are defined as "active" have the ability to provide nutrient retention and release (i. e., incorporate fertilizing capability) in addition to the other primary soil functions of the above mentioned inert soils.
It is known that nutrient retaining activity in natural soils is due to the presence of organic matter and clay components. Such components have charge sites suitable for ion exchange. Prior to release, the nutrient elements are held at the charge sites as "exchange ions." Recent introduction of ion exchange media (that are not normally found in natural soils) having a high exchange ion holding capacity have made feasible the development of active synthetic soil-fertilizers which can supply plant nutrients over a long period of time.
Mineral zeolites have been found to be a class of very useful ion exchange media. Many natural species are prevalent and numerous synthetic species have been made in the laboratory. Zeolites are hydrated aluminosilicates of alkali and alkaline-earth cations that possess infinite, three-dimensional crystal structures (i.e., tektosilicates). The primary building units of the zeolite crystal structure are (Al,Si)O.sub.4 tetrahedra. When Al.sup.3+ and sometimes Fe.sup.3+ substitute for Si.sup.4+ in the central cation position of the tetrahedron, a net-negative charge is generated. This negative charge is counterbalanced primarily by monovalent and divalent "exchange cations." Zeolites have shown the ability to exchange most of their constituent exchange cations as well as hydrate/dehydrate without major changes in the structural framework. Most zeolites have large channels and/or cages that allow exchange cations easy access to charge sites and provide unique cation selectivity.
The use of zeolites as a major soil component has a relatively recent past. U.S. Pat. No. 4,337,078 to Petrov et al. describes the use of a natural zeolite clinoptilolite with vermiculite and peat in a synthetic soil. The term zeoponics has been coined to describe synthetic soils containing zeolites in horticulture.
Agronomists and botanists have long recognized the vital function of sixteen nutrients needed by growing plants including the trace elements or micronutrients--zinc, chlorine, iron, manganese, copper, molybdenum and boron. It is also known that the optimal spectrum and concentration of micronutrients in a particular soil can vary depending on the plants being grown, soil properties, climate, and the stage of the plant growth cycle.
While most natural soils contain micronutrients at least to some extent and the overall need is small, depletion can occur with intensive agricultural activity. Even when the soil concentration is putatively adequate, other factors can prevent micronutrient uptake by the plant. Since micronutrients must be available as soluble ions, such ions can be immobilized in low solubility alkaline soils and/or can be trapped on clays or organic materials as insoluble complexes.
It has been common practice to supplement phosphorus-impoverished soil by using a mineral fertilizers such as rock phosphate or natural apatite Such minerals, however, do not supply the required micronutrients and can contain toxic elements such as fluorine and cadmium.
Rock phosphate as mined is relatively insoluble in water. Therefore, the raw product is generally pretreated to enhance phosphate solubility prior to use. Such processes, however, are considered too expensive for farmers in underdeveloped nations. Yet, fertilizer use is necessary to promote economic development. It has been suggested by Chesworth et al., Applied Clay Science, 2:291-297, 1987, Barbarick et al., Colorado State University Department of Agronomy Technical Bulletin No. TB88-1, June 1988 and Lai et al., Zeolites, 6:129-132, 1986 that a combination of natural untreated rock phosphate and an ion exchange medium such as a zeolite, which are both relatively abundant in underdeveloped regions of the world, can be made to increase the solubility of rock phosphate in the soil without pretreatment. The zeolite is thought to act as a sink for calcium cations and induce further dissolution of the rock phosphate.
Agriculture at lunar colonies will require development of artificial soils and fertilizers which perform the four primary functions of a natural soil (e. g., nutrient retention, aeration, moisture retention and mechanical support). It is also desirable that such artificial soils be manufactured substantially from lunar resources, provide an entire spectrum of essential nutrient elements and be substantially free of toxic elements.
Ming D. W., Lunar Base Agriculture Soils for Plant Growth, (Ming and Henniger, ed.), American society of Agronomy, Madison, Wis., 1989, pp. 93-106 discusses the use of zeolites in the manufacture of synthetic soils on the moon.
Lewis M. D. et al., Zeo-Agriculture: Use of Natural Zeolites in Agriculture and Aquaculture, (Pond and Mumpton ed.), Boulder, Colo.:Westview Press, 1983, pp. 105-111, describes the use of granulated clinoptilolite, ammonium-exchanged clinoptilolite and urea as nitrogen fertilizers.
Pirela, D. G. et al., Zeo-Agriculture: Use of Natural Zeolites in Agriculture and Aquaculture, (Pond and Mumpton ed.), Boulder, Colo.:Westview Press, 1983, pp. 113-122, describes the use of clinoptilolite in combination with nitrogen fertilization to increase plant growth.
Parham, W. E., Zeo-Agriculture: Use of Natural Zeolites in Agriculture and Aquaculture, (Pond and Mumpton ed.), Boulder, Colo.:Westview Press, 1983, pp. 283-285, surveys the use of natural zeolites in the agricultural arts.
Ferguson et al., Soil Science Society of America Journal, 51:231-234, 1987 describes ammonium retention in sand amended with clinoptilolite.
Ferguson et al., Agronomy Journal, 78:1095-1098, Nov-Dec, 1987 describes the growth of creeping bentgrass on a clinoptilolite amended sand.
Iskenderov et al., Occurance, Properties and Utilization of Natural Zeolites, (Kallo and Sherry ed.), Budapest:Akademiai Kiado, 1988 pp. 717-720, describes the utilization of natural zeolite in Azerbaijan for increasing wheat yield.
MacKown et al., Journal of American Soil Science Society, 49:235-238, 1985 describes the mobilization of ammonium nitrogen in a coarse textured soil amended with zeolite.
Allen E. R. et al., Agronomy Abstracts, p. 193, Nov. 27-Dec. 2, 1988 describes use of a zeolite-apatite substrate to supply nitrogen, phosphorus and potassium by ion exchange.
U.S. Pat. No. 3,958,973 to Roberts describes a micronutrient metal containing phosphate glass for fertilizer use. The glass is based on P.sub.2 O.sub.5 and the micronutrients are based on the metal oxide. A solubility control agent is said to be present to control the amount and rate of release.
U.S. Pat. No. 4,299,613 to Carderelli describes a polymeric composition incorporating essential plant growth compounds in ionic form. These compounds are said to be gradually, continuously and uniformly released over a long period of time in response to the presence of moisture.
U.S. Pat. No. 4,334,906 to Young describes a combination soil amendment and micronutrient source. The composition comprises highly porous sulfur particles having substantial internal surface area with the micronutrient source dispersed either throughout the particle matrix or over the interior surfaces.
U.S. Pat. No. 4,670,039 to Sjogren describes a controlled slow release fertilizer composition comprising an encapsulated fertilizer, carbon particles and plaster.
U.S. Pat. No. 4,557,749 to Berthet et al. describes a sealed container for a hydrosoluble fertilizer or agricultural product. The container has a wall made of a hydrophobic polymer diaphragm with hydrophilic inclusions which absorb water. Water passing into the container dissolves the fertilizer which is then desorbed into the medium being treated.
Other U.S. patents of interest include U.S. Pat. No. 4,994,100 to Sutton et al.; U.S. Pat. No. 4,507,139 to Sullivan et al.; U.S. Pat. No. 4,175,943 to Jordaan et al.; and U.S. Pat. No. 4,995,897 to Schramm et al.